CATV systems are well known for providing quality television viewing entertainment in rural areas where there is limited television reception and in high rise apartment buildings where the television recption in individual apartments is poor. Because of the large number of channels that may be accomodated in modern CATV systems, cable television is becoming increasingly popular even in areas where the television reception is good. First in apartment buildings and now more and more prevelant elsewhere, bidirectional CATV systems are being used for a variety of applications including security systems and educational programs. In a typical security system using a bidirectional CATV coaxial cable, video signals and interrogation signals are transmitted from the head end or a hub of the CATV system to a plurality of subscribers. At each subscriber location, one or more sensors such as an intrusion detector or a fire or smoke detector are connected to a transceiver that detects its unique address and responds to an interrogation signal. In an educational television system, students at various subscriber locations may be provided with equipment to respond to prompts from the television receiver.
Persons familiar with the state-of-the-art in CATV system upstream transport have come to realize the difficulties associated with providing this service reliably due mainly to multi-source noise build-up in the reverse direction. CATV systems are constructed very much like a tree in that from a single location, a trunk connects to many sub-branches and ultimately to a much higher number of subscriber bearing networks. The greater the number of subscribers, the greater the potential for noise accumulation at the point in the trunk system where ultimately all upstream signals combine. The noise is created in part by the electronic device required in the subscriber's location and in part by the vulnerability of subscriber interconnects or drops to interference from sources outside the cable plant. Unless controlled, the noise buildup in the system soon results in the system becoming unmanageable.
Schemes such as computer directed or signal triggered bridger switching have been devised to control upstream noise, but these schemes typically limit the number of data channels which can be simultaneously accomodated within a given bandwidth. While effective in reducing accumulated noise, switching introduced at the bridger station has the undesirable feature of limiting the time any given subscriber has access to the return circuit. It would, in fact, be impossible to supply a metropolitan area with home security using a sequentially polled system and simultaneously deliver uninterrupted video from a high school play without some form of over-ride to keep the school's upstream circuit active. In a densely populated area, it is not inconceivable for many users to demand simultaneous use of the network for an equal diversity of upstream functions.